I don't think modern (the last 20 years) treated lumber is all that toxic. They say you can use modern treated lumber in raised garden beds. However, it is still not allowed in Organic Farming.
Although I think OP has already bought some standard indoor non-treated lumber for his roosts, I just want to throw out another idea. Lately, I have been picking up pallets from our local Harbor Freight. Most of the pallets are normal size. However, some of those pallets are 8 and 10 foot long with excellent heat treated (HT) 2X4's as the stretchers/supports. As far as I know, the HT process only involves heat, and no chemicals. I would think a person could break down those pallets and use that wood in a chicken coop without any worry.
I would not use any pallets that have oil or chemical spills on the wood, but the Harbor Freight pallets I have been getting don't have any signs of that use. Anyway, if you are willing to put a little effort into breaking down the pallets, you can reclaim some useable wood from a pallet at zero cost.